The present invention is directed to improvements in microelectronic fuses and relates to surface mountable fuses employing only a single insulating member or chip, that can be directly soldered to a PC board.
Surface Mount Devices (SMDs) may be soldered directly to circuit boards and are preferred for use in circuit board mountings over through-hole axial devices. The surface attachment technology increases the packaging density and lowers the manufacturing and product cost. A unique characteristic of surface mounting is the ability for the chip and packaged components to be attached to both the component and solder sides of the board. The use of SMDs is becoming popular in consumer electronics.
The incorporation of the SMDs into all types of electronic devices created a need for SMD fuses. Surface mount fuses are used to prevent damage to electrical systems, caused by excessively large currents. The failure of a fuse can result in a fire or loss of costly equipment. It is conventional practice to employ a surface mount fuse in PC board circuits for protection from such excessively large currents.
There are several types of chips available for surface mounting. A first type is a wrap-around that has metallization around the full surface of the opposite ends of a rectangular chip. Another type, the flip chip, is terminated with metallization on a bottom side of the chip. There are also two types of internal constructions generally known for surface mount fuses. The first type is comprised of a thin wire fuse element spaced between two metal terminals. The wire is placed above the surface of the insulating body and connected to the leads or terminals by soldering, resistance welding, or wire bonding techniques. The complete device is enclosed in a plastic casing fabricated from thermoplastic materials by using injection molding techniques or by securing a cover to the base which encloses the fuse. Use of additional parts to provide for termination and enclosure to the fuse adds to the manufacturing complexity and ultimately to the cost of the fuse.
U.S. Pat. Nos. 5,162,773 to Shiozaki, 5,130,688 to Van Rietchoten, 4,612,529 to Gurevich et al., 4,547,830 to Yamauchi, 4,873,506 to Gurevich, 3,500,276 to Hingorany et al., 5,140,295 to Vermot-gaud et al., 5,166,656 to Badihi et al., and 4,563,666 to Borzoni all show variations of such a fuse. These patents disclose surface mount fuses with fuse elements comprised of a metal wire attached to prefabricated terminals by different means such as soldering, brazing, electrical or ultrasonic welding. The fuse elements are secured to the surface of the base, and thus must be enclosed by a sleeve, cover, or other enclosure to protect the PC board and the other components from the heat produced by the fuse elements, and the effects of the fuse if it should fail.
Thermoplastic materials often are used for the sleeve, body, and/or cover of the fuse. The thermoplastic materials are generally low temperature material which can melt and fail from temperatures above 300.degree. C. However, fuse elements can generate these temperatures during prolonged openings at low overloads. The use of these materials can thus lead to fuse failure.
A second type of SMD fuse is comprised of the thin or thick film elements deposited on the surface of a substrate using thin or thick film printing or sputtering techniques. These fuses have larger variances in their characteristics due to inability of the deposition techniques to provide consistency of the thickness. Thick film printing generally can provide control only within 30% of the expected value, and thin film deposition, without additional processing, generally can provide control only within 15% of the expected value. By adding substantial cost to photolithographic techniques, thin film printing or spottering can be improved to be within 5% of the expected value. The higher accuracy of the cross sectional area of conductor and its length is required to obtain repeatable fuse performance. These fuses have the disadvantage of being expensive to produce accurately.
The second kind of fuse also has a large surface contact area between the deposited metallized coating and the insulating member. The larger contact area can cause the heat generated by resistance heating of the fuse element to be conducted to the substrate material at a rate directly proportional to its size. This may delay the opening of the fuse and lead to device overheating. Excessive overheating of the fuse may cause a melting of the solder joints holding the fuse in place. The fuse may disconnect from the board and cause arcing external to a fuse joint. Overheating may result in damage of the PC board material in close proximity to a fuse and diffusion of the thin film terminations into the molten solder which may result in premature opening of the fuse.
There are numerous variations of the two types described, however all such varieties have drawbacks to a greater or lesser degree.